5.3 Lipid biosynthesis and degradation
3 min read•august 7, 2024
Lipid biosynthesis and degradation are crucial processes in plant metabolism. Plants use for , , and signaling. These processes involve complex enzymatic pathways and specialized organelles, allowing plants to efficiently manage their lipid resources.
occurs in the cytosol, while degradation happens in peroxisomes. Plants can convert lipids to sugars during seed germination. The modifies fatty acids, producing important signaling molecules for plant defense and communication.
Fatty Acid Synthesis
Acetyl-CoA Carboxylase and Fatty Acid Synthase
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- catalyzes the first committed step in fatty acid synthesis by converting acetyl-CoA to malonyl-CoA
- Requires biotin as a cofactor and is allosterically regulated by citrate (activator) and palmitoyl-CoA (inhibitor)
- is a multienzyme complex that catalyzes the synthesis of palmitate (16-carbon saturated fatty acid) from acetyl-CoA and malonyl-CoA
- Utilizes NADPH as a reducing agent
- Consists of two identical monomers, each containing multiple enzymatic domains (acyl carrier protein, condensing enzyme, reducing enzymes)
- Fatty acid synthesis occurs in the cytosol of plant cells and is a highly endergonic process requiring significant energy input (ATP and NADPH)
Triacylglycerols and Phospholipids
- (triglycerides) are the primary storage form of fatty acids in plant cells
- Consist of three fatty acid molecules esterified to a backbone
- Stored in specialized organelles called oil bodies or lipid droplets
- are the major structural components of cellular membranes (plasma membrane, organelle membranes)
- Amphipathic molecules with a hydrophilic head (phosphate group and a polar molecule like choline) and hydrophobic tails (fatty acids)
- Arrange into bilayers with hydrophobic tails facing inward and hydrophilic heads facing the aqueous environment
- Both triacylglycerols and phospholipids are synthesized in the endoplasmic reticulum from fatty acids and glycerol-3-phosphate
Fatty Acid Degradation
Beta-oxidation and Lipases
- is the primary pathway for fatty acid degradation in plant cells
- Occurs in peroxisomes and (specialized peroxisomes in germinating seeds)
- Fatty acids are broken down in a cyclic process, removing two carbons at a time as acetyl-CoA
- Acetyl-CoA can enter the citric acid cycle for energy production or be used for gluconeogenesis in germinating seeds
- are enzymes that catalyze the hydrolysis of ester bonds in lipids, releasing fatty acids and glycerol
- Play a crucial role in mobilizing stored lipids (triacylglycerols) during seed germination and seedling growth
- Also involved in lipid remodeling and signaling processes
Glyoxysomes and Ketone Bodies
- Glyoxysomes are specialized peroxisomes found in germinating seeds of oil-storing plants (castor bean, sunflower)
- Contain enzymes for beta-oxidation and the glyoxylate cycle, which allows the conversion of fatty acids into sugars for seedling growth
- Glyoxylate cycle bypasses the decarboxylation steps of the citric acid cycle, conserving carbon for gluconeogenesis
- (acetoacetate, beta-hydroxybutyrate) are water-soluble compounds produced from acetyl-CoA when fatty acid breakdown exceeds the capacity of the citric acid cycle
- Can serve as an alternative energy source for tissues during prolonged fasting or stress conditions
- Not as prevalent in plants as in animals, but can accumulate in certain situations (extended darkness, senescence)
Lipid Modification
Lipoxygenase Pathway
- The lipoxygenase pathway is a major route for the oxidative modification of polyunsaturated fatty acids (linoleic acid, linolenic acid) in plants
- Lipoxygenases are non-heme iron-containing enzymes that catalyze the oxygenation of polyunsaturated fatty acids, forming
- Multiple isoforms exist with different substrate specificities and cellular localizations
- Hydroperoxides can be further metabolized by various enzymes (allene oxide synthase, hydroperoxide lyase) to produce a diverse array of
- Oxylipins include (jasmonic acid), , and divinyl ethers
- The lipoxygenase pathway is involved in various aspects of plant growth, development, and stress responses
- Jasmonates are key signaling molecules in plant defense against herbivores and pathogens
- Green leaf volatiles (hexanal, hexenal) are important in plant-plant communication and plant-insect interactions
Key Terms to Review (27)
Abscisic acid: Abscisic acid (ABA) is a plant hormone that plays a crucial role in regulating various physiological processes, particularly during stress responses. It helps plants manage drought conditions, seed dormancy, and influences growth by acting as a signaling molecule that coordinates developmental processes.
Acetyl-CoA Carboxylase: Acetyl-CoA carboxylase is an essential enzyme that catalyzes the conversion of acetyl-CoA into malonyl-CoA, a crucial step in fatty acid biosynthesis. This enzyme plays a key role in lipid metabolism, as malonyl-CoA serves as the building block for fatty acid chains. The regulation of acetyl-CoA carboxylase is vital for maintaining the balance between lipid synthesis and degradation, influencing energy storage and membrane formation.
Beta-oxidation: Beta-oxidation is the metabolic process by which fatty acids are broken down in the mitochondria and peroxisomes to generate acetyl-CoA, which can then enter the Krebs cycle for energy production. This process is crucial for lipid degradation, allowing cells to utilize stored fats as an energy source, especially during periods of fasting or prolonged exercise.
Chloroplasts: Chloroplasts are specialized organelles found in plant cells and some protists that conduct photosynthesis, converting light energy into chemical energy stored in glucose. These structures are key players in plant physiology as they not only produce food for the plant but also generate oxygen, playing an essential role in the ecosystem.
Energy storage: Energy storage refers to the processes and systems that capture energy for use at a later time, allowing organisms to manage their energy supply efficiently. In the context of living organisms, this often involves the synthesis and breakdown of biomolecules that serve as reservoirs of energy, such as lipids. By converting excess energy into stable forms, organisms can ensure a steady supply for cellular functions during periods of low energy availability.
Ethylene: Ethylene is a plant hormone that plays a crucial role in regulating various physiological processes, including growth, development, and responses to environmental stimuli. It is known for its involvement in fruit ripening, leaf senescence, and stress responses, making it an essential factor in plant physiology.
Fatty acid synthase: Fatty acid synthase is a multi-enzyme complex responsible for the biosynthesis of fatty acids from acetyl-CoA and malonyl-CoA through a series of reactions. This enzyme plays a crucial role in lipid biosynthesis, facilitating the formation of long-chain fatty acids that are essential for various biological functions, including energy storage and membrane structure.
Fatty acid synthesis: Fatty acid synthesis is the biological process by which organisms create fatty acids, essential components of lipids, from acetyl-CoA and malonyl-CoA through a series of enzymatic reactions. This process occurs mainly in the cytoplasm and is crucial for producing energy storage molecules and membrane components, linking it directly to lipid biosynthesis and degradation.
Fatty acids: Fatty acids are carboxylic acids with long hydrocarbon chains, which can be saturated or unsaturated. They play a crucial role in lipid biosynthesis and degradation, serving as the building blocks of triglycerides and phospholipids, which are essential components of cellular membranes and energy storage molecules.
Glycerol: Glycerol is a simple polyol compound that serves as a backbone for the formation of triglycerides and phospholipids in lipid metabolism. It is an important three-carbon molecule that combines with fatty acids to form lipids, which play vital roles in energy storage, cellular structure, and signaling processes within cells.
Glyoxysomes: Glyoxysomes are specialized organelles found in plant cells that play a crucial role in the metabolism of fatty acids. They are particularly important during seed germination, where they convert stored lipids into carbohydrates through the glyoxylate cycle, enabling the seedling to grow until it can perform photosynthesis.
Green leaf volatiles: Green leaf volatiles (GLVs) are a group of volatile organic compounds released by plants when their leaves are damaged or stressed, often as a response to herbivory. These compounds play critical roles in plant communication, signaling neighboring plants to prepare their defenses and attracting natural predators of herbivores. Understanding the biosynthesis and degradation of these compounds sheds light on their function in plant physiology.
Hydroperoxides: Hydroperoxides are organic compounds that contain a peroxide functional group (-O-O-) attached to a hydrocarbon chain, which can significantly impact lipid biosynthesis and degradation processes. These compounds are formed during the oxidation of lipids and can serve as both signaling molecules and precursors for various reactive species in biological systems. Their formation and degradation play crucial roles in cellular signaling pathways and stress responses.
Jasmonates: Jasmonates are a class of plant hormones derived from fatty acids that play crucial roles in regulating plant growth, development, and defense mechanisms. They are involved in responses to various stresses, such as wounding, pathogen attacks, and environmental factors, and function as signaling molecules that help coordinate a plant's defensive strategies while also influencing growth processes.
Ketone bodies: Ketone bodies are water-soluble molecules produced by the liver during periods of low carbohydrate availability, such as fasting or prolonged exercise. They serve as an alternative energy source for tissues, especially during times when glucose is scarce, and include three primary compounds: acetoacetate, beta-hydroxybutyrate, and acetone. Their production and utilization highlight the body's ability to adapt to different metabolic states.
Lipases: Lipases are enzymes that catalyze the hydrolysis of lipids, breaking down fats into glycerol and fatty acids. These enzymes play a crucial role in lipid metabolism, facilitating both lipid biosynthesis and degradation, which is essential for energy production and maintaining cellular functions.
Lipid bodies: Lipid bodies are organelles found in plant cells that store lipids, particularly triacylglycerols, and play a crucial role in lipid metabolism. These structures are important for energy storage and contribute to the overall metabolic balance within the cell by regulating lipid biosynthesis and degradation processes.
Lipid catabolism: Lipid catabolism is the biological process through which lipids, such as fats and oils, are broken down into smaller molecules to release energy. This process is essential for organisms as it provides ATP and other metabolic intermediates that can be used in various cellular functions, linking it closely with the processes of lipid biosynthesis and degradation.
Lipid droplet formation: Lipid droplet formation refers to the process by which cells create specialized storage organelles that consist mainly of triglycerides and cholesterol esters surrounded by a monolayer of phospholipids and proteins. These droplets serve as reservoirs for energy storage and play crucial roles in lipid metabolism, signaling, and cellular homeostasis.
Lipoxygenase pathway: The lipoxygenase pathway is a metabolic route that converts polyunsaturated fatty acids into various bioactive compounds known as leukotrienes and lipoxins, which play crucial roles in plant signaling and defense responses. This pathway is essential for the production of jasmonates, which are important signaling molecules that regulate plant growth, development, and stress responses.
Membrane structure: Membrane structure refers to the arrangement and composition of biological membranes, primarily composed of lipid bilayers with embedded proteins, which serve as barriers and facilitators of transport in and out of cells. This structure is crucial in maintaining cellular integrity and function, influencing various physiological processes such as signaling, energy transfer, and cellular communication.
Oxylipins: Oxylipins are a diverse group of bioactive lipid molecules derived from the oxidation of fatty acids. These compounds play crucial roles in plant physiology, serving as signaling molecules that mediate responses to various environmental stresses and developmental processes. Their biosynthesis is tightly linked to the metabolism of membrane lipids, reflecting the intricate relationship between lipid biosynthesis and degradation.
Phospholipids: Phospholipids are a class of lipids that are major components of all cell membranes, forming the lipid bilayer. They consist of two fatty acid tails and a phosphate group attached to a glycerol backbone, which creates a hydrophilic (water-attracting) head and hydrophobic (water-repelling) tails. This unique structure allows them to self-assemble into bilayers, providing a barrier that separates the internal environment of cells from the external surroundings.
Photorespiration: Photorespiration is a metabolic process in plants that occurs when the enzyme RuBisCO oxygenates ribulose-1,5-bisphosphate instead of carboxylating it, leading to the consumption of energy and release of CO2. This process typically takes place under high light intensity and low carbon dioxide concentrations, which can negatively impact plant productivity by reducing the efficiency of photosynthesis.
Signaling pathways: Signaling pathways are complex networks of interactions that transmit signals from the exterior of a cell to its interior, ultimately leading to a cellular response. These pathways play a crucial role in regulating various physiological processes, including lipid biosynthesis and degradation, by allowing cells to sense and respond to changes in their environment. They involve a series of molecular events, often initiated by ligands binding to receptors, that culminate in the activation of downstream effectors and transcription factors, resulting in specific gene expression and metabolic changes.
Transcription factors: Transcription factors are proteins that help regulate the expression of specific genes by binding to nearby DNA. They play a crucial role in controlling the timing and rate of transcription, influencing various biological processes such as development, hormone responses, and stress adaptation. By interacting with other proteins and signaling pathways, transcription factors can mediate complex cellular responses and cross-talk among different hormonal signals.
Triacylglycerols: Triacylglycerols, also known as triglycerides, are a type of lipid made up of three fatty acid chains esterified to a glycerol backbone. They serve as a major form of energy storage in plants and animals, providing a dense energy source when metabolized. Their synthesis and breakdown are crucial processes in lipid metabolism, impacting overall energy balance and cellular functions.